Cold pilger processes are known having a shaping of the roll calibration of the roll mandrel in the manner that diameter and course of the reduction in the wall proceed without interruption along a continuous curve. Methods are also known in which a reduction in diameter without mandrel contact is first effected and then predominantly a reduction of the wall. Furthermore, methods are described by which special re-shaping steps, for instance widening, are carried out in different re-shaping zones.
The methods of the prior art have the disadvantage that they do not permit the continuous feeding of fresh cooling and lubricating agent into the temporarily opened annular gap between roll mandrel and inner surface of the material being rolled. In other words, the lubricant which passes over the inner surface of the billet into the annular gap is not renewed during the passage of a tube cross-section through the re-shaping zone.
It should also be mentioned that the inner surface of the material being rolled is frequently substantially enlarged by the re-shaping. This can lead to the interruption of the lubricating film. Furthermore, in the case of large rolling outputs, the lubricating action can collapse entirely since, with certain tube materials, the development of heat is considerably increased by the re-shaping work. The result is a welding together of the tube and mandrel, leading to rejected tubes.
Additions of solids to the lubricant are intended to reduce the problem, but they lead to expensive methods of cleaning. The prior art is also unsatisfactory ecologically inasmuch as chlorine-paraffin oils are generally used for the rolling of the tube materials, so that any further development leading to better lubricating conditions should include the replacement of the presently used chlorine-paraffin oils by less objectionable products.
European Pat. No. 01 48 806, discloses a method permitting a feed of fresh oil into the temporary gap between the material being rolled and the roll mandrel. The disclosed method has the disadvantage that, for the stated purpose, the mandrel must be moved back and then forward upon each stroke of the stand, or that seals which are subject to injury are provided within the inside of the billet or tube.